1. Field of the Invention
The present invention relates to a dielectric filter, a dielectric duplexer, and a communication apparatus incorporating the same, in which a dielectric material is used in a resonator part.
2. Description of the Related Art
Generally, for example, when a dielectric duplexer is formed by disposing a plurality of dielectric resonators in a dielectric block, a plurality of resonant-line holes are arranged in the dielectric block to form resonant lines on the inner surfaces of the holes, by which there are provided a transmitting filter section, in which signals of a transmitting band are allowed to pass through and signals of a receiving band are attenuated, and a receiving filter section, in which signals of the receiving band are allowed to pass through and signals of the transmitting band are attenuated.
When the transmitting filter and the receiving filter are band-pass type filters, pass characteristics of the filters are as shown in FIGS. 14A and 14B. In this case, the symbol Tx denotes the pass characteristics of the transmitting filter, and the symbol Rx denotes the pass characteristics of the receiving filter. As indicated by hatching F1, F2, F3 and F4 in the figures, a maximum insertion loss in a transmitting band (F1) and a minimum insertion loss in a receiving band (F2) are determined as the characteristics of the transmitting filter, whereas a maximum insertion loss in the receiving band (F3) and a minimum insertion loss in the transmitting band (F4) are determined as the characteristics of the receiving filter. The transmitting filter and the receiving filter are designed so that they can satisfy these conditions.
However, the pass characteristics shown in FIGS. 14A and 14B are the characteristics at a specified temperature. In general, in dielectric filters and dielectric duplexers, the higher the temperature, the more the unloaded Q factor (Qo) of a resonator is deteriorated. This is due to the temperature characteristics of electrode materials. For example, in the case of silver or copper, conductivity decreases by approximately 2% with an increase of every 10xc2x0 C. The conductivity decrease of the electrode directly leads to the deterioration of Qo. As a result, the higher the temperature, the more the insertion loss of the filter is deteriorated.
In general, since the characteristics of a pass band are determined by a maximum insertion loss and a region specifying a frequency range (from one threshold frequency to the other threshold frequency) thereof, both shoulder portions of the pass band characteristics (the portions A and B shown in FIGS. 14A and 14B) are in proximity to the ends of the region. In addition, in the case of a duplexer, since a transmitting band and a receiving band are conventionally in proximity to each other, the shoulder portion in a range from the pass band to the attenuation band thereof is the closest to the end of a side close to the attenuation band in a region specifying the maximum insertion loss and the frequency range thereof (the position indicating the maximum insertion loss and the frequency range is hereinafter referred to as a xe2x80x9cthresholdxe2x80x9d).
For example, the filter (the transmitting filter) on the lower-frequency side of the pass band has a threshold on the higher-frequency side of the pass band, as shown at a portion A FIG. 14A. The filter (the receiving filter) on the higher-frequency side of the pass band has a threshold on the lower-frequency side of the pass band, as shown at a portion B.
In this case, when the temperature of the dielectric duplexer is increased, Qo of a resonator is deteriorated due to the above-described reason, by which insertion losses are increased as indicated by dotted lines in FIG. 14A. Furthermore, when the temperature is over a certain degree, both the high-frequency side shoulder portion of the pass characteristics of the transmitting filter and the low-frequency side shoulder portion of the pass characteristics of the receiving filter go beyond the maximum insertion loss at each of the thresholds.
Although the example shown in FIG. 14A illustrates a case where the permittivity-temperature characteristics of the dielectric material are fixed (in which permittivity does not change regardless of temperature changes), when the dielectric material has permittivity-temperature characteristics, as shown in FIG. 14B, according to the inclination of the characteristics, the pass characteristics move toward either the high-frequency side or the low-frequency side. For example, when the higher the temperature is the lower the permittivity so that the resonant frequency is increased, pass characteristics as indicated by dotted lines in FIG. 14B are exhibited. In this case, the shoulder portion of the pass characteristics of the receiving filter having an attenuation band on the lower-frequency side goes beyond the maximum insertion loss of the threshold, as shown at the portion B. Furthermore, as shown in FIG. 14A, the waveform of the pass characteristics does not only move toward the lower direction, but it moves toward the right-lower slanting direction in the figure. Therefore, the problems described above occur even at relatively low temperatures.
The above-described problems occur not only in the case of a dielectric duplexer, but the problems also occur in the case of a single dielectric filter in which a threshold is in proximity to the shoulder portion where insertion losses increase in a region from the pass band to the attenuation band.
To overcome the above described problems, preferred embodiments of the present invention provide a dielectric filter, a dielectric duplexer, and a communication apparatus incorporating the same, in which deterioration of insertion-loss characteristics with respect to temperature changes is improved so that good characteristics are exhibited over a wide range of temperatures. In this invention, even if temperature changes occur in a dielectric filter or a dielectric duplexer, a waveform exhibiting the pass characteristics of the device is moved in such a manner that the waveform does not go beyond a threshold determined by a maximum insertion loss and a threshold frequency thereof.
One preferred embodiment of the present invention provides a dielectric filter having an attenuation band in proximity to a pass band, a threshold-frequency position of a determined maximum insertion loss being arranged close to a shoulder portion of a waveform exhibiting pass characteristics in which insertion losses increase in a region from the pass band to the attenuation band. In this dielectric filter, temperature characteristics of a dielectric material are determined in such a manner that the shoulder portion moves toward the attenuation-band direction according to an increase and decrease in temperature. With this arrangement, even if the pass characteristics of the filter change according to an increase and decrease in temperature, since the shoulder portion in the region from the pass band to the attenuation band moves in such a manner that they avoid a threshold, by which specified characteristics can be maintained.
The above described dielectric filter may be formed by a plurality of dielectric resonators, at least one of the dielectric resonators being a trap resonator forming an attenuation pole in a region from the shoulder portion to the attenuation band. In addition, the temperature characteristics of the dielectric material are determined in such a manner that resonant-frequency changes with respect to temperature changes in the trap resonator are smaller than those with respect to temperature changes in the other dielectric resonator. With this arrangement, attenuation characteristics near the attenuation pole are fixed regardless of temperature changes, so that specified attenuation characteristics can be maintained.
Furthermore, the plurality of the dielectric resonators may be integrally molded or integrally fired as a single dielectric block. Although there is a problem in that, if a dielectric filter is formed by combining discrete dielectric resonators, an error in the arrangement occurs, since the difference in the temperature characteristics of a dielectric material cannot be judged from the appearance, the present invention can solve the problem.
The above described dielectric filter may be a band pass filter formed by a plurality of dielectric resonators in which the pass band is used as the range of a resonant frequency. With this arrangement, the insertion loss of the pass band is smaller and the insertion loss at the shoulder portion of the pass band adjacent to the attenuation band can be maintained at a low level over a wide range of temperatures.
The dielectric filter may be a band block filter formed by a plurality of dielectric resonators in which the attenuation band is used as the range of a resonant frequency. With this arrangement, a large amount of attenuation in the attenuation band can be obtained, and at the same time, the insertion loss at the shoulder portion of the pass band adjacent to the attenuation band can be maintained at a low level over a wide range of temperatures.
Another preferred embodiment of the present invention provides a dielectric duplexer including the above-described two dielectric filters, one of the two filters being a dielectric filter in which the low-frequency band of the filter is an attenuation band and the high-frequency band thereof is a pass band, and the other filter being a dielectric filter in which the low-frequency band of the filter is a pass band and the high-frequency band thereof is an attenuation band. With this arrangement, in both filters, the shoulder portion of the pass characteristics in the region from the pass band to the attenuation band does not go beyond a maximum insertion loss over a wide range of temperatures, by which the functions of the duplexer can be maintained. In addition, in this dielectric duplexer, when the two dielectric filters are integrally molded or integrally fired by a single dielectric block, the above-described error in the arrangement does not occur.
Yet another preferred embodiment of the present invention provides a communication apparatus including one of the dielectric filter and the dielectric duplexer described above, which is disposed in a high-frequency circuit section. With this arrangement, a communication apparatus in which a specified signal-processing function of the high-frequency circuit section can be maintained over a wider range of temperatures.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.